A switch-mode power converter is a power processing device that converts an input voltage or current source waveform into a specified output voltage or current waveform. Fundamental to the operation of switch-mode converters are controllers, which manage the operation of the converter by controlling the on and off periods of switches employed therein. Generally, controllers are coupled between the output and input of the converter in a feedback loop configuration (also referred to as a control loop or closed-control loop). Typically, the controller measures an energy level at the output (e.g., power, voltage, current, etc.) and based on the quantity of the output, dynamically increases or decreases switch on-time in the converter to maintain a constant output to a load.
Today, controllers in switch-mode power converters are primarily composed of interconnected analog circuits. Analog circuitry is undesirable for numerous reasons, for example: (1) Each controller requires hundreds of interconnected discrete components to obtain the desired functionality of a single controller. Hundreds of discrete components translates into large inventory costs, complicated and expensive manufacturing processes. (2) Analog controllers tend to require a fairly extensive area of power supplies to account for numerous discrete components comprising the controller, which is a disadvantage in today's power system industry where there is an ever increasing desire to minimize the size of power supplies. (3) Analog hardware is generally fixed and inflexible. Modifications to analog hardware controllers typically require a complete redesign of the entire controller. (4) Analog circuitry is subject to packaging and component defects especially as the number of components increases. (6) Analog circuitry is sensitive to noise.
As is evident from the foregoing, there are many problems associated with analog control circuits used in switch-mode power converters. Despite the numerous limitations of analog circuitry, however, it remains the choice for use in a majority of commercially manufactured switch-mode converters today, because there are no suitable technology design alternatives available for switched-mode converters.
Digital controllers have been successfully employed in place of analog controllers in slower power system designs such as Uninteruptable Power Supplies (UPSs), motor drives and three-phase rectifiers, which typically require response times in the range of 10-to-500 Hz or two-to-six milli seconds. In such devices, microprocessors and digital signal processors (DSPs) are typically able to execute commands in an allotted amount of time consistent with controlling the power system.
Nevertheless, microprocessors and DSPs are not viable alternatives to analog controllers for use with switch-mode converters today. For instance, microprocessors and DSPs are generally not capable of delivering required speeds and bandwidths as analog converters. A typical commercial AC-to-DC or DC-to-DC switch mode converter requires a response time of approximately 20 .mu.s and bandwidth requirements in the range of 5 kHz-to-100 kHz. Controllers for switch-mode converters must account for sampling within a switching cycle, which is two orders of magnitude faster than slower power system designs mentioned above. Such response times are easily obtained by analog circuitry. Microprocessors and Digital Signal Processor (DSPs), are generally not capable of matching the wide bandwidth requirements in their control loops to meet the fast transient requirements and high switching frequencies of such switch-mode converters.
Moreover, overcoming these limitations would require employing cost prohibitive processor schemes, when compared to the cost of analog controllers used today. For instance, a 60 watt DC-to-DC converter with analog control circuitry may cost around a $100 U.S. dollars, whereas the cost of a DSP alone may cost around $150.00 U.S. dollars. High performance DSPs are simply too expensive for use with switch-mode power converters when compared to low cost items like analog controllers for use with such controllers.
Attempts have been made to employ Application Specific Integrated Circuits (ASICs) as a digital alternative to analog circuitry in the control loop of switch-mode converters. ASICs provide parallel and pipelined processing which boosts speed considerably. When applied in a high volume product, an ASIC is also cost-effective. Nevertheless, ASICs, like analog circuitry, lack flexibility, since they are not re-programmable and are generally fixed designs. Once an ASIC controller design is fabricated, it is extremely difficult to reconfigure. Furthermore, ASICs do not lend themselves to lower volume products in terms of cost benefits and require considerable expense and effort to design and build.
The vast majority of commercial switch-mode power supplies available today still employ conventional analog circuitry in their control loops. Accordingly, what is needed is a cost effective and flexible digital alternative to conventional analog control circuitry in switch-mode converters.